This invention relates to a composite screen adapted for use in pressure filters and pressure screens such as for separating solids from liquids, such as for separating cellulose used in making paper from waste water, and screens for screening impurities from or grading paper making stock.
Pressure filters with screening members, such as cylindrical screens, are used in the paper and pulp preparation industry for recovering usable paper fibers from a thin slurry or liquor containing such fibers. Such screens commonly operate within a closed pressure vessel, in which foil members rotate at relatively high speed and in close proximity to a screen inlet surface, to prevent the surface from blocking while extracting liquid through the screen and retaining the screened or material fibers at the inlet surface for discharge in a thickened condition. Typical apparatus for this purpose is the Liqui-Filter screening apparatus as sold by Thermo Black Clawson Inc., 605 Clark Street, Middletown, Ohio 45042.
Rotating foil pressure filters commonly use a generally cylindrical screen in which either the inside or the outside surface is the filtration surface. More commonly, the inside surface is the inlet surface of the screen, and one or more impulse foil members are mounted on a drum or on arms extending from a central shaft. The foil members move in close proximity to the inlet surface to maintain the pulp suspension in a fluidized condition at such surface. Such screening apparatus is a closed pressure unit, commonly operating at a pressure differential across a cylindrical screen in the range of about 30 to 40 psi.
In filtering to remove or retain cellulose fibers or the like, commonly cylindrical screens, or screen baskets as they are sometimes called, are employed with small openings or very narrow slots. Typically, small openings are drilled through a wall of thin plate material, while the material is in a flat condition. The flat plate material, after drilling, is then rolled into a cylinder and welded, and end rings welded on for strength and for support within the screening apparatus.
Typically, screen openings are formed by electron beam drilling a 5/16" thick plate material, and may be as small as 0.004" but more commonly in the range of 0.006" to 0.008" in diameter. Relatively high foil speeds are employed, such as in the range of about 5000 feet per minute or higher at the inner inlet screen surface to maintain a fluidized condition of the material at that surface for screening and/or thickening and separation.
Screen baskets, as previously described, are relatively expensive to make primarily because of the labor involved in making many very small openings through the screen wall. Thus, these baskets are disproportionately expensive compared to their size and to the cost of the overall screening apparatus. Also they are, as are all screen baskets, subject to failure and destruction by reason of metal fatigue or uncontrolled over pressurization or other factors and, in which event, a comparatively costly component can quickly become destroyed and unusable. Accordingly, there is a need for a less costly and equally effective or more effective alternative construction for screen baskets.
In the filter screening apparatus, the perforations or holes must be sufficiently small as to prevent the desired fibers from going through the holes, while permitting the liquid component to be extracted through the openings in the screen basket. Therefore, these openings must be quite small, as previously defined, so that a major portion of the paper fibers are retained.
The concepts of this invention may also be employed in the manufacture of a conventional screen basket for apparatus, as shown for example, in U.S. Pat. No. 3,849,302 in which undesirable components in a paperstock slurry are screened and eliminated from the slurry. These may be particles of dirt, trash and lump like particles that have not been fully defibered. Thus, in conventional paperstock screening, the openings in the screen are sufficiently large so as to permit the paper fibers to pass through while blocking or preventing the flow of undesirable products that may be in the paper pulp suspension. In other words, the "accepts" flow through the screen while the "rejects" move along the screen surface to an outlet. Therefore, such screens commonly have openings, either in the form of slots or holes, that are larger than those typically used in filter screens where the fiber is blocked by the filter, and such openings may be in the range of as small as 0.010" width or smaller or up to 0.030" width or larger, depending upon the particular location or function of the pressure screen, and the material being screened. Such screens are likewise subject to high stresses by reason of the pressure gradient across the screens and the rotating impulse foils that are used in close proximity to the screen surface for aiding in the screening function.